This invention relates to skid control systems and more particularly to braking pressure control valves for use in these systems.
To stop a moving vehicle in the shortest possible distance, the braking pressure applied to the brakes of the vehicle must be maintained at an optimum level. This optimum level of braking pressure will vary according to several factors, including the coefficient of friction between the vehicle wheels and the particular road upon which the vehicle is traveling. Anti-skid systems have been developed for automatically varying the braking pressure in accordance with these factors.
Many anti-skid systems include three basic components: a wheel speed sensor component for detecting the speed of a decelerating wheel, a logic component for receiving wheel speed data from the sensor and comparing it to an optimum wheel deceleration rate, and a brake pressure regulating valve component for reducing the braking pressure applied to the wheel brake upon receipt of an electrical signal from the logic component signifying that the wheel is experiencing a skid or an impending skid situation. The brake pressure regulating valve is also adapted to reapply braking pressure to the brake upon removal of the electrical signal by the logic component.
The effectiveness of the anti-skid system is dependent largely upon the responsiveness and other capabilities of the pressure regulating valve employed therewith. Several types of brake pressure regulating valves have been developed for use in anti-skid system. Many of these valves are made operative to regulate braking pressure by the use of solenoid assemblies. Solenoid assemblies are generally operative to move only between "on" and "off" positions. Thus many anti-skid valves are limited in the anti-skid mode of operation to an intermittent opening and closing of the brake line.
For example, a solenoid in such an anti-skid valve receiving a signal from the anti-skid logic component that a skid situation at the wheel is impending can cause the valve to block additional brake fluid and can release braking pressure at the brakes. When the impending skid situation is eliminated, the solenoid assembly allows the valve to reapply braking pressure.
Because an immediate and full reapplication of braking pressure occurs and because time is of the essence, the above-described solenoid actuated valves may tend to overshoot the optimum braking pressure. This can immediately cause a second impending skid situation and subsequent removal of braking pressure. Several rapid applications, exhausts, and reapplications of braking pressure can result before the valve produces the optimum braking pressure. This "hunting" by the valve for the optimum braking pressure can be extremely inefficient as to the distance and time required to stop a vehicle because the deceleration rate of the wheel must be permitted to increase after each skid or impending skid. The lost braking time in compensating for each skid situation can be critical, especially in emergency situations.
Anti-skid valves having double solenoid assemblies have been utilized to produce a multiple increment reduction and reapplication of braking pressure. Valves of this type can limit the amount of braking pressure applied or exhausted to smaller amounts. After a wheel skid, overshooting of the optimum braking pressure is not as severe because pressure can be reapplied in smaller increments until a maximum is attained. A rapid series of pressurizations and exhausting of brake fluid, however, is still the normal operating procedure. Further, the cost of the additional components of the valve to create this effect can be a disadvantage and may not be worth the increased efficiency attained.